Ranking Salt Solutions by pH
6-2: Ranking Salt Solutions by pH
In this assignment you will be asked to rank aqueous solutions of acids, bases, and salts in order of increasing pH. This is most easily done by first identifying the strong acids that have the lowest pH, the strong bases that have the highest pH, and the neutral solutions that have a pH near 7. The weak acids will have a pH between 1 and 6 and the weak bases between 8 and 14. The exact order of weak acids and weak bases is determined by comparing the ionization constants (Ka for the weak acids and Kb for the weak bases). After ranking the pH of these solutions, you will then test your predictions in the laboratory.
1. Arrange the following 0.1 M solutions in order of increasing pH and state why you placed each solution in that position: NaCH3COO, HCl, HCN, NaOH, NH3, NaCN, KNO3, H2SO4, NH4Cl, H2SO3,, NaHCO3, Na3PO4 and CH3COOH.
In order of increasing pH:
Here are my predictions:
- H2SO4 –
- HCL –
- H2SO3 – Weak Acid;
- CH3COOH – Weak Acid;
- NH4Cl – Weak Acid;
- HCN- Weak Acid;
- KNO3 – This is neutral
- NaCH3COO – This is a weak base;
- NaHCO3 –Weak Base;
- NaCN – Weak Base;
- NH3 – Weak Base;
- Na3PO4 - Weak Base;
- NaOH -
Once you have predicted the nature of each salt solution, you will use Virtual ChemLab to confirm your prediction. Each solution must be approximately 0.1 M for your comparisons to be valid. Most of the solutions in the Stockroom are approximately 0.1 M already. Two solutions will need to be diluted and three solutions will need to be prepared from solid salts. One of these salt solutions is already prepared and on the lab bench ready for you to measure the pH.
2. Start Virtual ChemLab and select Ranking Salt Solutions by pH from the list of assignments. The lab will open in the Titrations laboratory.
3. On the stir plate, there will be a beaker of 0.10 M ammonium chloride (NH4Cl) that has already been prepared. The pH meter has been calibrated and is in the beaker. Record the pH of the NH4Cl solution in the data table on the following page. When finished, drag the beaker to the red disposal bucket, and drag the bottle of NH4Cl to the stockroom counter.
Virtual ChemLab: General Chemistry Laboratories, Student Lab Manual/Workbook, v.2.5, Third Edition, by Brian F. Woodfield, Matthew C. Asplund, and Steven Haderlie. Published by Prentice Hall. Copyright © 2006 by Pearson Education, Inc. Acid-Base Chemistry
4. Click in the Stockroom to enter. Double-click on the NH4Cl bottle to return it to the shelf and then double-click on the NaHCO3 and KNO3 bottles to move them to the Stockroom counter. Return to the laboratory
5. Open the beaker drawer (click on it) and drag a beaker to the spotlight next to the Balance. Click and drag the bottle of NaHCO3 and place on the spot light near the balance. Click in the Balance area to zoom in. Place a weigh paper on the balance and tare the balance. Open the bottle by clicking on the lid (Remove Lid). Pick up the Scoop and scoop up some salt by dragging the Scoop to the bottle and then down the face of the bottle. Each scoop position on the face of the bottle represents a different size scoop. Pull the scoop down from the top to the second position (approximately 0.20 g) and drag it to the weigh paper in the balance until it snaps into place. Releasing the scoop places the sample on the weigh paper. Now drag the weigh paper from the balance to the beaker until it snaps into place and then empty the salt into the beaker. Return to the laboratory and drag the beaker to the stir plate.
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